This invention relates to a security system for monitoring defined zones and, more particularly, for monitoring areas such as exits in shops or stores.
Perimeter surveillance or monitoring systems are used in a variety of applications. These systems generally comprise an antenna in the monitored zone and a receiver/transmitter connected to the antenna. Security tags are attached to individual items to be monitored. The security tags include a coil which tunes the tag to a specific resonant frequency. The antenna is also tuned to the same frequency. The antenna transmits a signal at short intervals. The tag, when in the vicinity of the antenna or the monitored zone, will oscillate in response to this received signal. These oscillations can be received by the antenna and be used to detect the presence of the tag in the monitored zone.
In some cases, the security tag resonant circuit is comprised of a fuse element such that a signal of higher energy than that employed for detection causes the fusible link of the resonant circuit to be destroyed to thereby deactivate the tuned circuit so that detection is no longer possible. In these circumstances, a deactivating device is located at a checkout area in a retail establishment or the like and is operable to produce the high energy fields for deactivating the tuned circuit of the security tag. Thus, allowing the article to be removed from the monitored area without triggering the alarm.
In order to keep the cost of the security tags as low as possible, the resonant circuit normally comprises an etched metal on a flexible substrate, such as plastic or cardboard. Thus, the resonant frequency of the security tags are not very accurate. In order to accommodate this, the transmitter is normally swept between two frequencies. Such systems operate from approximately 7.7 MHz to 8.7 MHz and are known as 8.2 MHz systems. In the swept system, the antennas must transmit continuously and, in the case where there are multiple antennas installed close to each other, each must be synchronized to transmit in a phase. This generally requires that each of the antennas are interconnected, which is problematic, particularly when a large area is to be monitored.
Alternatively, a fixed frequency system as for example, described in U.S. Pat. No. 5,471,196 may be utilized. These systems generally operate in the 2 MHz range and require accurately tuned tags. In the fixed frequency system, the transmitter emits a short burst of energy for approximately a few milliseconds. The system then waits for radiation emitted by the tags before transmitting another burst of energy. Timing of the burst may be synchronized to the mains frequency, which makes it possible to operate a number of systems, all having short bursts at different times. Thus, for a 1.2 mS burst synchronized to the mains frequency, makes it possible to operate up to twelve systems, all having short bursts at different times. Thus, although the frequency system overcomes a problem of synchronizing multiple systems, it still suffers from the disadvantages of requiring accurately tuned tags.
Accordingly, there is a need for a system which mitigates the above disadvantages of present systems.
This invention thus seeks to provide a security system that may operate with tags that are not accurately tuned, thus, operating like a swept frequency system while offering advantages of a fixed frequency system.
A further object of the invention is to provide a security system with increased detection range while minimizing the possibility of accidental tag deactivation by the system.
In accordance with this invention there is provided a security system for monitoring the presence of one or more objects in a monitored zone, said system comprising an antenna and at least one tuned resonant circuit associated with one or more objects to be monitored;
a transmitter coupled to said antenna operable to transmit at one of a plurality of frequencies in response to a frequency selection signal, said transmitter operating said antenna for cyclic transmission of each of said plurality of frequencies within a repeatable transmission/receive sequence interrupted by pause intervals;
a receiver coupled to said antenna and for producing a detection signal for stopping said cycling of said transmitter frequencies in response to a signal received at said receiver from said resonant circuit thereby causing said transmitter to transmit only at a single frequency when a tag is detected.
In accordance with a further aspect of this invention there is provided a security system for monitoring the presence of one or more objects in a monitored zone, said system comprising an antenna and at least one turned resonant circuit associated with one or more objects to be monitored;
a transmitter coupled to said antenna operable to transmit at one of a plurality of frequencies in response to a frequency selection signal;
a receiver coupled to said antenna and for producing a detection signal and
a level detection circuit coupled to an output of said receiver for decreasing the output power of said transmitter when said received signal is above a predetermine level.